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United States Patent |
6,066,419
|
Wu
,   et al.
|
May 23, 2000
|
Method for monitoring dosage/focus/leveling
Abstract
A method for monitoring dosage/focus/leveling is provided. A control wafer
is provided and divided into several regions. Five of the regions near the
center of the wafer are used to monitor normally. Other regions are used
as dummy shots. When a situation of a stepper changes greatly, the
dosage/focus/leveling of the control wafer is monitored using the dummy
shots. In monitoring exposure dosage, the middlemost region is monitored.
One of the five regions, which is the most central, is exposed with a low
exposure energy to enhance sensitivity of critical dimension versus
energy. Many points with small areas are developed in the centermost
region to take sufficient samples. Since the developed points are close,
effects from the nonuniformity of development and from the nonuniformity
of the photoresist layer are prevented. In focus/leveling monitoring, a
curve diagram of exposure dosage versus critical dimension is provided. An
exposure parameter is taken at a range of the curve with a large slope.
The focus/leveling is monitored at the other four regions near the
middlemost region.
Inventors:
|
Wu; Cheng-Kuan (Taipei, TW);
Fang; Te-Yang (Taipei, TW)
|
Assignee:
|
United Semiconductor Corp. (Hsinchu, TW)
|
Appl. No.:
|
270278 |
Filed:
|
March 16, 1999 |
Foreign Application Priority Data
Current U.S. Class: |
430/30 |
Intern'l Class: |
G03F 009/00 |
Field of Search: |
430/30
|
References Cited
U.S. Patent Documents
5288572 | Feb., 1994 | Giapis et al. | 430/30.
|
Primary Examiner: Young; Christopher G.
Attorney, Agent or Firm: Huang; Jiawei
J.C. Patents
Claims
What is claimed is:
1. A method for monitoring dosage/focus/leveling, comprising the steps of:
providing a controlling wafer comprising a first region, second regions,
first dummy shots and second dummy shots, wherein the first region has
exposure points;
applying a low exposure energy to the exposure points of the first region
to monitor exposure dosage; and
applying a high exposure energy to the second regions to monitor focus and
leveling.
2. The method according to claim 1, wherein an anti-reflecting layer is
formed on the controlling wafer.
3. The method according to claim 2, wherein the anti-reflecting layer is an
organic layer.
4. The method according to claim 2, wherein the anti-reflecting layer is an
inorganic layer.
5. The method according to claim 1, wherein a monitoring range of focus is
between about -0.3 .mu.m and about 0.3 .mu.m.
6. The method according to claim 5, wherein the first dummy shots are
monitored when the monitoring range of focus is smaller than -0.3 .mu.m or
larger than 0.3 .mu.m.
7. The method according to claim 1, wherein a monitoring range of leveling
is between about -15 .mu.rad and about 15 .mu.rad.
8. The method according to claim 7, wherein the second dummy shots are
monitored when the monitoring range of leveling is smaller than -15
.mu.rad or larger than 15 .mu.rad.
Description
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the priority benefit of Taiwan application serial
no. 87116427, filed Oct. 2, 1998, the full disclosure of which is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to a method for monitoring semiconductor
integrated circuits (ICs), and more particularly to a method for
monitoring dosage/focus/leveling at steppers while performing a
photolithography process.
2. Description of the Related Art
Photolithography process plays an important role in semiconductor
fabrications. For example, a metal oxide semiconductor (MOS) fabrication
needs several photolithography processes to pattern several desired
patterns and dope active regions. So, the number of photolithography
processes taken usually represents the complexity of a fabrication
process. The number of masks used also tells the complexity of a
fabrication, because each photolithography process needs one mask.
According to reduction of the size of integrated circuits, process windows
of a photolithography process, such as exposure energy (EE) and depth of
focus (DOF), diminish. The baseline, such as exposure dosage, accuracy of
focus or leveling of a chip, of a stepper must be controlled carefully
during commercial production, so that an efficient monitoring system is
required to ensure processes that are performed correctly.
A conventional method for measuring an exposure dosage is using a
photo-speed monitor stepper to control the exposure dosage. When a
photoresist layer on a wafer is exposed completely, that means the
exposure dosage applied on the wafer is enough. The wafer is divided into
several regions. Exposure dosage of a first region is lower than of a
second region. Exposure dosage of the second region is lower than of a
third region. The regions are checked one by one to determine whether the
regions are developed. If some regions are not developed, the exposure
dosage is insufficient. On the other hand, of regions are developed
completely, the exposure dosage is sufficient. An exposure dosage of a
stepper can be adjusted according to the results described above. However,
to observe exposure is not easy due to nonuniformity of the photoresist
layer and nonuniformity of developing.
Conventionally, a stepper laser beam is used to monitor the depth of focus
of the stepper. Pattern length is measured under different defocus
conditions according optical diffraction theory. The pattern length is the
longest when the focus is correct. However, this process occupies the
stepper for about 10 minutes.
A wafer placed on the stepper may be sloped so that the best focus is
different at different position of the wafer. A conventional method uses
an auto focus beam detect stepper to level the stepper stage. This process
occupies the stepper for about 40 minutes.
These methods for monitoring dosage/focus/leveling are complicated and are
performed sequentially. The methods require three wafers and parameters.
Furthermore, the methods have to use a stepper so that they are unsuited
for daily commercial production monitoring.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a method for
simultaneously monitoring dosage/focus/leveling. An exposure pattern is
designed on a control wafer. Different positions of the control wafer are
applied with different exposure parameters.
It is therefore another object of the invention to provide a monitoring
method using a scanning electron microscope (SEM). The method is not
performed on a stepper and so is suitable for daily monitoring.
The invention achieves the above-identified objects by providing a method
for monitoring dosage/focus/leveling. A control wafer is provided and
divided to several regions. Five of the regions near the center of the
wafer are used for normal monitoring. Other regions are used as dummy
shots. When a situation of a stepper changes eminently, the
dosage/focus/leveling of the control wafer is monitored using the dummy
shots. In monitoring exposure dosage, the middlemost region is monitored.
One of the five regions, which is the middlemost, is applied a low
exposure energy to enhance sensitivity of critical dimension versus
energy. Many points with small areas are developed at the middlemost
region to take enough samples. Since the developed points are close,
effects from the nonuniformity of development and from the nonuniformity
of the photoresist layer are prevented. In focus/leveling monitoring, a
curve diagram of exposure dosage versus critical dimension is provided. An
exposure parameter is taken at a range of the curve with a large slope.
The focus/leveling is monitored at the other four regions near the
middlemost region.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features, and advantages of the invention will become
apparent from the following detailed description of the preferred but
non-limiting embodiments. The description is made with reference to the
accompanying drawings in which:
FIG. 1A is a schematic diagram showing monitoring regions of a control
wafer of one preferred embodiment of the invention;
FIG. 1B is a magnification of a schematic diagram of the second of the
monitoring regions shown in FIG. 1A;
FIG. 2 is a curve diagram of exposure dosage versus critical dimension
(CD);
FIG. 3 is a curve diagram of defocus versus CD;
FIG. 4 is a curve diagram of CD difference versus leveling sensitivity of
x-axis of one preferred embodiment of the invention;
FIG. 5 is a curve diagram of CD difference versus leveling sensitivity of
y-axis of one preferred embodiment of the invention;
FIG. 6 is a curve diagram showing a leveling repeatability curve of one
preferred embodiment of the invention; and
FIG. 7 is a curve diagram showing a focus repeatability curve of one
preferred embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In FIG. 1A, a control wafer 10 is provided. An anti-reflecting coating
(ARC) layer 11 is formed on the control wafer 10 to prevent light
reflecting from the control wafer 10. A material of that ARC layer 11
comprises an organic ARC layer or an inorganic ARC layer. A first region
12 and four, second regions 14 are defined on the control wafer 10 and
near the center of the control wafer 10. The first region 12 and the
second regions 14 are monitored under normal stepper conditions. Normal
conditions means that focus difference is between -0.3 to 0.3 .mu.m and
leveling difference is between -15 to 15 .mu.rad . A scanning electron
microscope (SEM) is used to monitor so that stepper operation time is not
used. Several exposure points 16 are formed in the first region 12 using
small reticle blind to take samples. Since the exposure points 16 are
close in the first region 12, effects from nonuniformity of the
photoresist layer and from nonuniformity of the development can be
prevented.
Furthermore, the first dummy shots 18a, 18b, 18c, 18d and second dummy
shots 20a, 20b are set at the other region of the control wafer 10. These
dummy shots 18a, 18b, 18c, 18d, 20a and 20b are monitored when the
condition of the stepper changes are great, for example, the focus
difference is larger than 0.3 .mu.m and the leveling difference is larger
than 15 .mu.rad. The first dummy shot 18a is monitored when the x-axis
leveling difference is larger that 15 .mu.rad. The first dummy shot 18b is
monitored when the x-axis leveling difference is smaller than -15 .mu.rad.
The first dummy shot 18c is monitored when the y-axis leveling difference
is larger than 15 .mu.rad. The first dummy shot 18d is monitored when the
y-axis leveling difference is smaller than -15 .mu.rad. The second dummy
shot 20a is monitored when the focus difference is larger than 0.3 .mu.m.
The second dummy shot 20b is monitored when the focus difference is
smaller than -0.3 .mu.m.
FIG. 1B is a schematic, blown-up diagram showing the second region 14 in
the FIG. 1A. There are four detecting points 141, 142, 143 and 144 at the
four corners of the second region 14 and a detecting point 145 at the
center of the second region 14. The central detecting point is used to
monitor the focus difference. The four detecting point 141, 142, 143, 144
are used to monitor the leveling difference. CD differences detected at
the four detecting points 141, 142, 143, 144 are used in an equation
(143+144)-(141+142)
to obtain the degree of x-axis slope. The CD difference are used in an
equation
(141+143)-(142+144)
to obtain the degree of y-axis slope.
In FIG. 2, the first region 12 is used to monitor exposure dosage. An
exposing process is performed at the first region 12 with a low exposure
energy. In the low-energy range of a curve shown in the FIG. 2, the slope
of the curve is steeper than other range of the curve so that the
sensitivity of CD is increased. An exposure level is observed during a
constant period. A low exposure level or an increase in CD means that
stepper exposure energy is lower than a predetermined energy. On the other
hand, a high exposure level or a decrease in CD means that the stepper
exposure energy is higher than the predetermined energy.
In FIG. 3, an exposure energy of curve a is higher than that of curve b.
The exposure energy of curve b is higher than that of curve c. The
exposure energy of curve c is higher than that of curve d. An exposure
parameter is chosen within a range with the steepest slope of the curves.
The sensitivity of CD difference within the range is better than in other
ranges. For example, an exposure parameter at point A of the curve b is
chosen. The sensitivity of point A is represented as
(4.0-3.3)/(1.5-1.2) =0.23 .mu.m/0.1 .mu.m.
The result means that the CD difference is 0.23 .mu.m when focus difference
is 0.1 .mu.m so that the focus is monitored easily by large change in the
CD difference. As shown in FIG. 3, the preferred range for monitoring
focus is between about -0.3-0.3 .mu.m (-1.2.+-.0.3 .mu.m) so that the
region of the curve b, which has a greatly changing slope, is selected.
FIG. 4 is a curve diagram of CD difference versus leveling sensitivity of
the x-axis of the invention. FIG. 5 is a curve diagram of CD difference
versus leveling sensitivity of the y-axis of the invention. The range of
leveling difference of the x-axis and the range of leveling difference of
the y-axis are the same and between about -15-15 .mu.rad. The range of the
CD difference of x-axis is between 0.15 .mu.m when leveling difference of
the x-axis changes in every 5 .mu.rad. The CD difference of y-axis is
between 0.10 .mu.m when leveling difference of the y-axis changes in every
5 .mu.rad. In the invention, the sensitivity of focus can achieve about
0.1 .mu.m. The monitoring range of focus is between about -0.3-0.3 .mu.m.
The leveling sensitivity can achieve about 5 .mu.rad. The monitoring range
of leveling is between about -15-15 .mu.rad. These ranges cover variations
of a machine to properly produce devices with small line width. The method
provided by the invention is properly used in I-line steppers and deep-UV
steppers.
In FIG. 6, LVL-X represents a leveling curve of the x-axis. LVL-Y
represents a leveling curve of the Y-axis. The monitoring result shows
that the CD difference is small that 0.04 .mu.m (the leveling difference
is smaller than 1.5 .mu.rad) according the method of the invention. FIG. 7
is a focus repeatability curve. The monitoring result shown that the focus
difference is between -0.026 .mu.m-0.026 .mu.m according to the method of
the invention.
A feature of the invention is a method for simultaneously monitoring
dosage/focus/leveling. The method only requires a controlling wafer for
monitoring. The controlling wafer is divided into several regions, which
have difference exposure parameters. The controlling wafer is analyzed
using SEM so that stepper operational time is not occupied for routine
detection.
Another feature of the invention is that a monitoring range covers both
slight and large difference using dummy shots of the controlling wafer
while the machine is off-center.
While the invention has been described by way of example and in terms of a
preferred embodiment, it is to be understood that the invention is not
limited thereto. To the contrary, it is intended to cover various
modifications and similar arrangements and procedures, and the scope of
the appended claims therefore should be accorded the broadest
interpretation so as to encompass all such modifications and similar
arrangements and procedures.
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